any ideas on spark plug type and heat range got 598 ci single dominators in 38 top gun ant sug thanks

NGK br 6 fs

How about the type heads you have and comp.ratio probably alum. with a crush washer type plug and a flat top piston or inverted dome you can start with a R5673A-8 this is an NGK I use this plug on all my SC and natural engines and is safe to start with as long as you are not trying to cover up a problem with A/F ratio or idle problems. 92 octane and timing about 35 degrees btdc.